8 W. M. DAVIS 



postulated. Here the flat interstream uplands cannot be reduced 

 to sharp crests because they are not Hfted high enough to be con- 

 sumed by the wasting of the valley-side slopes while the slopes 

 are still steep; the uplands are therefore transformed into rounded 

 ridges, and after upheaval ceases the rounded ridges are slowly 

 worn down. Although the sharp-ridge stages of the first ideal 

 cycle are wanting in this second ideal cycle, the earher and the 

 later stages of both cycles are much alike. In the third ideal 

 cycles, a slow upheaval is postulated : here the valleys are deepened 

 about as fast as the land is raised, they are widened faster than 

 they are deepened, the flat interstream uplands are degraded almost 

 as fast as they are raised, and sharp forms of strong reHef are 

 therefore unattainable however long upheaval continues. (It may 

 be here noted that valleys which, either by reason of slow upheaval 

 or of weak rocks, are opened with graded slopes as fast as they are 

 deepened, may be described as "born mature"; also that a region 

 which represents the third ideal cycle may be described as "old 

 from birth." Furthermore, the late stage of Penck's third cycle 

 will be very similar to the late stages not only of his second and first 

 cycles, but also of a cycle of erosion introduced by an upheaval so 

 rapid that little erosion is accompKshed while it is taking place. 

 The chief difference between the late stages of these four ideal 

 cycles would seem to be that the streams would be best adjusted 

 to weak structures by the process of river capture in the last men- 

 tioned cycle of very rapid upheaval, and least adjusted in Penck's 

 third cycle of very slow upHft; this being for the reason that the 

 more rapid the upheaval, the better the opportunity for large streams 

 to cut down their valleys to a greater depth than that of small- 

 stream vaUeys.) 



Uncertain application of the first ideal cycle to the Alps. — Although 

 no separate examples of mountain forms are adduced in illustration 

 of the second and third ideal cycles, it is pointed out that in a 

 single mountain system, such as the Alps, these two cycles may 

 perhaps find exempHfication near the mountain margin, where 

 upheaval was presumably slow and small, and where the ridges 

 are usually rounded. At the same time the first ideal cycle is 

 thought to apply to the center of the Alpine system, where upheaval 



